Process to prepare a microcystalline wax and a middle distillate fuel

ABSTRACT

The invention relates to a process to prepare a microcrystalline wax and a middle distillate fuel by (a) hydrocracking/hydroisomerizing a Fischer-Tropsch product, wherein the weight ratio of compounds having at least 60 or more carbon atoms and compounds having at least 30 carbon atoms in the Fischer-Tropsch product is at least 0.2 and wherein at least 30 wt % of compounds in the Fischer-Tropsch product have at least 30 carbon atoms, (b) performing one or more distillate separations on the effluent of step (a) to obtain a middle distillate fuel fraction and a microcrystalline wax having an initial boiling point of between 500 and 600° C.

The invention is directed to a process to prepare a Fischer-Tropschderived microcrystalline wax.

A process route is disclosed for the preparation of Fischer-Tropschderived microcrystalline wax products by the so-called Shell MiddleDistillate Synthesis (SMDS) process is described in “The Markets forShell Middle Distillate Synthesis Products”, Presentation of Peter J. A.Tijm, Shell International Gas Ltd., Alternative Energy ' 95, Vancouver,Canada, May 2-4, 1995. This publication describes the preparation ofvarious grades of wax products having congealing points ranging from 31to 99° C. The disclosed process involves a Fischer-Tropsch synthesisstep wherein a waxy product is obtained. This product is firsthydrogenated and the hydrogenated product is separated by means ofdistillation into the various wax product grades. The product with thehighest congealing point is referred to as SX100.

Said presentation also discloses a process to prepare middle distillatesby hydrocracking/hydroisomerisation of the Fischer-Tropsch synthesisproduct.

A disadvantage of the SX100 grade or similar commercial Fischer-Tropschderived grades having a congealing point as determined by ASTM D 938 ofbetween 85 and 120° C. is that they are too hard to be used in someapplications. The hardness of a wax may be measured by the IP 376method. Typical PEN values at 43 ° C. as obtained using this method oncommercially available Fischer-Tropsch derived SX100 waxes are between0.2 and 0.6 mm.

An almost similar process as the SMDS process disclosed in saidpresentation is disclosed in the recently published WO-A-0174969. In thedisclosed process a Fischer-Tropsch product is subjected to ahydro-processing step at low conversion. The waxy products as obtainedin the examples of said publication are characterized by means of aNeedle Penetration Value according to ASTM D-1321. Because thetemperature at which said value is measured is not provided noassessment of the softness of these products can be made. Furthermore amelting point is mentioned without providing a method on how thisproperty was measured.

A disadvantage of the disclosed process in WO-A-0174969 or the disclosedSMDS process line-up is that a dedicated wax hydroconversion step isneeded to prepare the wax products next to a dedicated middle distillatehydroconversion step to prepare middle distillates from aFischer-Tropsch synthesis product.

The object of the present invention is to integrate the process ofpreparing soft waxes having a high congealing point with the productionof middle distillate fuels having good cold flow properties.

The following process achieves this object. Process to prepare amicrocrystalline wax and a middle distillate fuel by.

-   (a) hydrocracking/hydroisomerisating a Fischer-Tropsch product,    wherein weight ratio of compounds having at least 60 or more carbon    atoms and compounds having at least 30 carbon atoms in the    Fischer-Tropsch product is at least 0.2 and wherein at least 30 wt %    of compounds in the Fischer-Tropsch product have at least 30 carbon    atoms,-   (b) performing one or more distillate separations on the effluent of    step (a) to obtain a middle distillate fuel fraction and a    microcrystalline wax having an initial boiling point of between 500    and 600° C.

Applicants found that by performing thehydro-cracking/hydroisomerisation step with the relatively heavyfeedstock a process is obtained wherein in one hydrocracking step bothmiddle distillates and a microcrystalline wax are obtained in a highyield. A further advantage of said process is that the fraction obtainedboiling between said middle distillates and the microcrystalline wax isvery suited as a lubricating base oil precursor. By dewaxing saidfraction excellent quality base oils may be obtained.

The process of the present invention results in middle distillateshaving exceptionally good cold flow properties. These excellent coldflow properties could perhaps be explained by the relatively high ratioiso/normal and especially the relatively high amount of di- and/ortrimethyl compounds. Nevertheless, the cetane number of the dieselfraction is more than excellent at values far exceeding 60, often valuesof 70 or more are obtained. In addition, the sulphur content isextremely low, always less than 50 ppmw, usually less than 5 ppmw and inmost case the sulphur content is zero. Further, the density ofespecially the diesel fraction is less than 800 kg/m³, in most cases adensity is observed between 765 and 790 kg/m³, usually around 780 kg/m³(the viscosity at 100° C. for such a sample being about 3.0 cSt).Aromatic compounds are virtually absent, i.e. less than 50 ppmw,resulting in very low particulate emissions. The polyaromatic content iseven much lower than the aromatic content, usually less than 1 ppmw.T95, in combination with the above properties, is below 380° C., oftenbelow 350° C.

The process as described above results in middle distillates havingextremely good cold flow properties. For instance, the cloud point ofany diesel fraction is usually below −18° C., often even lower than −24°C. The CFPP is usually below −20° C., often −28° C. or lower. The pourpoint is usually below −18° C., often below −24° C.

The relatively heavy Fischer-Tropsch product used in step (a) has atleast 30 wt %, preferably at least 50 wt %, and more preferably at least55 wt % of compounds having at least 30 carbon atoms. Furthermore theweight ratio of compounds having at least 60 or more carbon atoms andcompounds having at least 30 carbon atoms of the Fischer-Tropsch productis at least 0.2, preferably at least 0.4 and more preferably at least0.55. Preferably the Fischer-Tropsch product comprises a C₂₀+ fractionhaving an ASF-alpha value (Anderson-Schulz-Flory chain growth factor) ofat least 0.925, preferably at least 0.935, more preferably at least0.945, even more preferably at least 0.955.

The initial boiling point of the Fischer-Tropsch product may range up to400° C., but is preferably below 200° C. Preferably any compounds having4 or less carbon atoms and any compounds having a boiling point in thatrange are separated from a Fischer-Tropsch synthesis product before theFischer-Tropsch, synthesis product is used in step (a). In addition tothe Fischer-Tropsch product also other fractions may be additionallyprocessed in step (a). Possible other fractions may suitably be anyexcess microcrystalline wax as obtained in step (b) or off-spec base oilfractions if base oils are also prepared in said-process.

Such a Fischer-Tropsch product can be obtained by any process, whichyields a relatively heavy Fischer-Tropsch product. Not allFischer-Tropsch processes yield such a heavy product. An example of asuitable Fischer-Tropsch process is described in WO-A-9934917 and inAU-A-698392. These processes may yield a Fischer-Tropsch product asdescribed above.

The Fischer-Tropsch product will contain no or very little sulphur andnitrogen containing compounds. This is typical for a product derivedfrom a Fischer-Tropsch reaction, which uses synthesis gas containingalmost no impurities. Sulphur and nitrogen levels will generally bebelow the detection limits, which are currently 5 ppm for sulphur and 1ppm for nitrogen.

The Fischer-Tropsch product may optionally be subjected to a mildhydrotreatment step in order to remove any oxygenates and saturate anyolefinic compounds present in the reaction product of theFischer-Tropsch reaction. Such a hydrotreatment is described inEP-B-668342. The mildness of the hydrotreating step is preferablyexpressed in that the degree of conversion in this step is less than 20wt % and more preferably less than 10 wt %. The conversion is heredefined as the weight percentage of the feed boiling above 370° C.,which reacts to a fraction boiling below 370° C. After such a mildhydrotreatment lower boiling compounds, having four or less carbon atomsand other compounds boiling in that range, will preferably be removedfrom the effluent before it is used in step (a).

The hydrocracking/hydroisomerisation reaction of step (a) is preferablyperformed in the presence of hydrogen and a catalyst, which catalyst canbe chosen from those known to one skilled in the art as being suitablefor this reaction. Catalysts for use in step (a) typically comprise anacidic functionality and a hydrogenation/dehydrogenation functionality.Preferred acidic functionality's are refractory metal oxide carriers.Suitable carrier materials include silica, alumina, silica-alumina,zirconia, titania and mixtures thereof. Preferred carrier materials forinclusion in the catalyst for use in the process of this invention aresilica, alumina and silica-alumina. A particularly preferred catalystcomprises platinum supported on a silica-alumina carrier. If desired,applying a halogen moiety, in particular fluorine, or a phosphorousmoiety to the carrier, may enhance the acidity of the catalyst carrier.Examples of suitable hydrocracking/hydro-isomerisation processes andsuitable catalysts are described in WO-A-0014179, EP-A-532118,EP-A-666894 and the earlier referred to EP-A-776959.

Preferred hydrogenation/dehydrogenation functionality's are Group VIIInoble metals, for example palladium and more preferably platinum. Thecatalyst may comprise the hydrogenation/dehydrogenation active componentin an amount of from 0.005 to 5 parts by weight, preferably from 0.02 to2 parts by weight, per 100 parts by weight of carrier material. Aparticularly preferred catalyst for use in the hydroconversion stagecomprises platinum in an amount in the range of from 0.05 to 2 parts byweight, more preferably from 0.1 to 1 parts by weight, per 100 parts byweight of carrier material. The catalyst may also comprise a binder toenhance the strength of the catalyst. The binder can be non-acidic.Examples are clays and other binders known to one skilled in the art.

In step (a) the feed is contacted with hydrogen in the presence of thecatalyst at elevated temperature and pressure. The temperaturestypically will be in the range of from 175 to 380° C., preferably higherthan 250° C. and more preferably from 300 to 370° C. The pressure willtypically be in the range of from 10 to 250 bar and preferably between20 and 80 bar. Hydrogen may be supplied at a gas hourly space velocityof from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. Thehydrocarbon feed may be provided at a weight hourly space velocity offrom 0.1 to 5 kg/l/hr, preferably higher than 0.5 kg/l/hr and morepreferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbonfeed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500Nl/kg.

The conversion in step (a) as defined as the weight percentage of thefeed boiling above 370° C. which reacts per pass to a fraction boilingbelow 370° C., is at least 20 wt %, preferably at least 25 wt %, butpreferably not more than 80 wt %, more preferably not more than 70 wt %.The feed as used above in the definition is the total hydrocarbon feedfed to step (a), thus also any optional recycle to step (a).

In step (b) one or more distillate separations are performed on theeffluent of step (a) to obtain at least one middle distillate fuelfraction and a micro-crystalline wax having an initial boiling point ofbetween 500 and 600° C. Suitably more middle distillate fuel fractionsare recovered from the effluent of step (a). Preferably at least two ofthe possible naphtha, kerosene or gas oil fractions are recovered fromthe product of step (a). Most preferably a gas oil fraction is isolatedhaving the above described cold flow properties. This distillateseparation is preferably performed by means of a distillation at aboutatmospheric conditions, preferably at a pressure of between 1.2-2 bara.The microcrystalline wax is preferably isolated from the bottom productas obtained in the atmospheric distillation by means of a distillationperformed at near vacuum conditions. This atmospheric bottom productpreferably boils for at least 95 wt % above 370° C. The vacuumdistillation is suitably performed at a pressure of between 0.001 and0.1 bara. The wax is preferably obtained as the bottom product of such adistillation. The distillate fractions as obtained in such adistillation may be recycled to step (a) or used to prepare lubricatingbase oils. This fraction may be further processed on site or sold as awaxy raffinate product. This product can be transported by for exampleship or trains to base oil production facilities elsewhere. This (baseoil precursor) fraction as obtained in said vacuum distillationpreferably has a T10 wt % boiling point of between 200 and 450° C. and aT90 wt % boiling point of between 300, and preferably between 400 and550° C.

The vacuum distillation of step (b) is preferably operated such that thedesired congealing point of the microcrystalline wax is obtained.

The soft microcrystalline wax as obtained with the above process haspreferably a congealing point as determined by ASTM D 938 of between 85and 120 and more preferably between 95 and 120° C. and a PEN at 43° C.as determined by IP 376 of more than 0.8 mm and preferably more than 1mm. The wax is further characterized in that it preferably comprisesless than 1 wt % aromatic compounds and less than 10 wt % naphtheniccompounds, more preferably less than 5 wt % naphthenic compounds. Themol percentage of branched paraffins in the wax is preferably above 33and more preferably above 45 and below 80 mol % as determined by C13NMR. This method determines an average molecular weight for the wax andsubsequently determines the mol percentage of molecules having a methylbranch, the mol percentage of molecules having an ethyl branch, the molpercentage of molecules having a C3 branch and the mol percentage havinga C4+ branch, under the assumption that each molecule does not have morethan one branch. The mol % of branched paraffins is the total of theseindividual percentages. This method calculated the mol % in the wax ofan average molecule having only one branch. In reality paraffinmolecules having more than one branch may be present. Thus the contentof branched paraffins determined by different method may result in adifferent value.

The oil content as determined by ASTM D 721 is typically below 10 wt %and more preferably below 6 wt %. If lower oil contents are desired itmay be advantageous to perform an additional de-oiling step. De-oilingprocesses are well known and are for example described in Lubricant BaseOil and Wax Processing, Avilino Sequeira, Jr, Marcel Dekker Inc., NewYork, 1994, pages 162-165. After de-oiling the wax preferably has a oilcontent of between 0.1 and 2 wt %. The lower limit is not critical.Values of above 0.5 wt % may be expected, but lower values can beachieved depending on the method in which the wax is obtained. Mostlikely the oil content will be between 1 and 2 wt %. The kinematicviscosity at 150° C. of the wax is preferably higher than 8 cSt and morepreferably higher than 12 and lower than 18 cSt.

The invention will be illustrated with the following non-limitingexamples.

Example 1

The C₅-C₇₅₀° C.+ fraction of the Fischer-Tropsch product, as obtained inExample VII using the catalyst of Example III of WO-A-9934917 wascontinuously fed to a hydrocracking step (step (a)). The feed containedabout 60 wt % C₃₀+ product. The ratio C₆₀+/C₃₀+ was about 0.55. In thehydrocracking step the fraction was contacted with a hydrocrackingcatalyst of Example 1 of EP-A-532118.

The effluent of step (a) was continuously distilled to give lights,fuels and a residue “R” boiling from 370° C. and above. The yield of gasoil fraction on fresh feed to hydrocracking step was 43 wt %. Theproperties of the gas oil as obtained are presented in Table 1. The mainpart of the residue “R” was recycled to step (a) and a remaining partwas separated by means of a vacuum distillation into a microcrystallinewax having the properties as listed in Table 2. The fraction ofmicrocrystalline wax obtained relative to the feed to the vacuumdistillation was 63.2 wt %.

The conditions in the hydrocracking step (a) were: a fresh feed WeightHourly Space Velocity (WHSV) of 1.02 kg/l.h, recycle feed WHSV of 0.31kg/l.h, hydrogen gas rate=1000 Nl/kg, total pressure=40 bar, and areactor temperature of 329° C. TABLE 1 Gas oil properties Cloud Point−20 CFPP −21 Pour Point < −24 Normals (wt %) 21.3 Iso's (wt %) 78.7Mono-methyl 39.5 Di-methyl 25.5 Others 13.8 Density (kg/l) 0.78 Cetane(D976m) 77 Cetane (D4737m) 85 T95 360

TABLE 2 Product of SX100* Paraflint H1** Example 1 Congealing point 97.3100 99 (ASTM D 938; ° C.) Drop melting point 110.0 113.5 112.3 (ASTM D127) (° C.) PEN at 25° C. (IP 376) (mm) 0.1  0.1 11.4 PEN at 43° C. 0.4 0.4 17.6 PEN at 65° C. 1.1  1.7 >20 Oil content (ASTM D 721; wt %) <0.1Not measured 4.6 Kinematic viscosity at 7.97 Not measured 13.9 150° C.(ASTM D 445) Micro-crystalline structure by Yes Yes Yes microscopicobservation*SX100 is a Fischer-Tropsch wax as marketed by Shell Malaysia bhp**Paraflint H1 is a Fischer-Tropsch derived wax marketed by SchumannSasol

1. A Process to prepare a microcrystalline wax and a middle distillatefuel by (a) hydrocracking-hydyroisomerizing a Fischer-Tropsch product,wherein the product has a weight ratio of compounds having at least 60or more carbon atoms and compounds having at least 30 carbon atoms of atleast 0.4 and wherein at least 30 wt % of compounds in theFischer-Tropsch product have at least 30 carbon atoms and wherein theconversion in step (a) is between 25 and 70 wt %, (b) performing one ormore distillate separations on the effluent of step (a) to obtain amiddle distillate fuel fraction and a microcrystalline wax having aninitial boiling point of between 500° C. and 600° C.
 2. The process ofclaim 1, wherein at least 50 wt % of compounds in the Fischer-Tropschproduct have at least 30 carbon atoms.
 3. The process of claim 1,wherein the microcrystalline wax as obtained has a congealing point ofbetween 95-120° C. and a PEN at 43° C. as determined by IP 376 of morethan 0.8 mm.
 4. The process of claim 3, wherein the PEN at 43° C. ismore than 1.0 mm.
 5. The process of claim 1, wherein the wax obtained instep (b) is subjected to an additional de-oiling step to obtain a waxhaving an oil content of between 0.1 and 2 wt %.
 6. The process of claim2, wherein the microcrystalline wax as obtained has a congealing pointof between 95-120° C. and a PEN at 43° C. as determined by IP 376 ofmore than 0.8 mm.
 7. The process of claim 2, wherein the wax obtained instep (b) is subjected to an additional de-oiling step to obtain a waxhaving an oil content of between 0.1 and 2 wt %.
 8. The process of claim3, wherein the wax obtained in step (b) is subjected to an additionalde-oiling step to obtain a wax having an oil content of between 0.1 and2 wt %.
 9. The process of claim 4, wherein the wax obtained in step (b)is subjected to an additional de-oiling step to obtain a wax having anoil content of between 0.1 and 2 wt %.